In recent years, the identification of mesenchymal stem cells, chiefly obtained from bone marrow, has led to advances in tissue regrowth and differentiation. Such cells are pluripotent cells found in bone marrow and periosteum, and they are capable of differentiating into various mesenchymal or connective tissues. For example, such bone-marrow derived stem cells can be induced to develop into myocytes upon exposure to agents such as 5-azacytidine (Wakitani et al., Muscle Nerve, 18(12), 1417-26 (1995)). It has been suggested that such cells are useful for repair of tissues such as cartilage, fat, and bone (see, e.g., U.S. Pat. Nos. 5,908,784, 5,906,934, 5,827,740, 5,827,735), and that they also have applications through genetic modification (see, e.g., U.S. Pat. No. 5,591,625). While the identification of such cells has led to advances in tissue regrowth and differentiation, the use of such cells is hampered by several technical hurdles. One drawback to the use of such cells is that they are very rare (representing as few as 1/2,000,000 cells), making any process for obtaining and isolating them difficult and costly. Of course, bone marrow harvest is universally painful to the donor. Moreover, such cells are difficult to culture without inducing differentiation, unless specifically screened sera lots are used, adding further cost and labor to the use of such stem cells. U.S. Pat. No. 6,200,606 by Peterson et al., describes the isolation of CD34+ bone or cartilage precursor cells (of mesodermal origin) from tissues, including adipose.
There remains a need for a more readily available source for large numbers of stem cells, particularly cells that can differentiate into multiple lineages of different germ layers, and that can be cultured without the requirement for costly prescreening of culture materials.
Other advances in tissue engineering have shown that cells can be grown in specially-defined cultures to produce three-dimensional structures. Spacial definition typically is achieved by using various acellular lattices or matrices to support and guide cell growth and differentiation. While this technique is still in its infancy, experiments in animal models have demonstrated that it is possible to employ various acellular lattice materials to regenerate whole tissues (see, e.g., Probst et al. BJU Int., 85(3), 362-7 (2000)). A suitable lattice material is secreted extracellular matrix material isolated from tumor cell lines (e.g., Engelbreth-Holm-Swarm tumor secreted matrix—“matrigel”). This material contains type IV collagen and growth factors, and provides an excellent substrate for cell growth (see, e.g., Vukicevic et al., Exp. Cell Res, 202(1), 1-8 (1992)). However, as this material also facilitates the malignant transformation of some cells (see, e.g., Fridman, et al., Int. J. Cancer, 51(5), 740-44 (1992)), it is not suitable for clinical application. While other artificial lattices have been developed, these can prove toxic either to cells or to patients when used in vivo. Accordingly, there remains a need for a lattice material suitable for use as a substrate in culturing and growing populations of cells.